The present invention generally relates to a travelling carriage driving apparatus, and more particularly to a full-size contact type scanner in which a full-size contact sensor is movably provided.
Conventionally, a reduction optical system scanner as shown in FIG. 1 is known as a scanner for optically reading pictures or images on a document. Referring to FIG. 1, light emitted from a light source 1 is projected onto a document 3 placed on a contact glass 2. The light reflected on the document 3 passes through first to fifth mirrors 4, 5, 6, 7 and 8, and an image formation lens 9, which forms a reduced image on a photoelectric conversion element 10 such as a charge-coupled device image sensor. The document 3 and the photoelectric conversion element 10 are placed in respective fixed positions. The light source 1 and the first mirror 4 are mounted on a first travelling carriage 11. The second and third mirrors 5 and 6 are mounted on a second travelling carriage 12. As is well known, the first and second travelling carriages 11 and 12 are moved in the speed ratio 2:1.
In the above-mentioned reduction optical system scanner where the document 3 and the photoelectric conversion element 10 are fixedly provided, an image can be correctly formed even when the first and second mirrors 4 and 5 move in a rickety state occurring in a direction perpendicular to the drawing sheet. Thus, it is not necessary to construct guide means for guiding the movement of the first and second travelling carriages 11 and 12 with high precision. For this reason, the guide means may be constructed with such a simple structure that the first and second travelling carriages 11 and 12 slide on poles or plate-shaped guides.
However, the reduction optical system itself provided in the reduction optical system scanner is large in size, and therefore the entire reduction optical system scanner is large.
Recently, a full-size contact type scanner as shown in FIG. 2 has attracted attention. Referring to FIG. 2, light emitted from a light source 15 is projected onto a document 14 placed on a contact glass 13, and is reflected thereon. A reflected light passes through a full-size image formation element 16, and forms an image on a full-size contact sensor 17 such as a charge-coupled device image sensor.
In full-scale type scanners such as the above, the document is held stationary, while the full-size contact sensor 17 is mounted on a travelling carriage 18 together with the light source 15 and the full-size image formation element 16, and is moved during the scan. If an error in position in the direction perpendicular to the drawing sheet (lateral vibration) occurs in the travelling carriage 18 during its movement, an error occurs in position between the document 14 and the full-size contact sensor 17 in the main scanning direction. This causes a jitter, which causes deformation of read-out images. The deformation increases with an increase of the resolution (such as an increase of 400 dpi.fwdarw.600 dpi.fwdarw.800 dpi). From this viewpoint, the straightness of the movement is required for full-size type image sensors. A stricter straightness is required, as the read pixel density is increased. For this reason, it is necessary to use precise guide means for moving the travelling carriage 18.
Improved travelling carriage driving systems are disclosed in Japanese Laid-Open Patent Application No. 61-18256 and Japanese Laid-Open Utility Model Application NO. 62-112255. Each of these documents proposes a driving system in which driving wires coupled to a travelling carriage are stretched on both sides thereof. However, the movement of a scanner fastened to the travelling carriage depends on a state of stretched wires or bearings. Therefore, it is very difficult to move the travelling carriage with high straightness. An improved guide is proposed in Japanese Laid-Open Patent Application No. 61-18256. However, the proposed guide is constructed by mechanical components, and therefore abrasion occurs therein with use. From this reason, it is very difficult to ensure the straightness of the movement of the travelling carriage simply by using mechanical components. Particularly, the travelling carriage used in the scanner is frequently reciprocated. Therefore, the occurrence of abrasion is a serious problem to be overcome.